This invention relates to bar code readers, and more particularly to a bar code reader for reading, using a laser beam, a bar code on a remote surface.
It has become common practice for sales and inventory control purposes, etc., to print a bar code on the surface of a product which may indicate manufacturer name, product name, price and manufacturing data. The bar code is read, using an optical bar code reader, at appropriate time for obtaining the coded information.
Bar code readers are generally either a so called "standing" type or a "handheld" type. The standing type reads a bar code attached to an object when the object and bar code are passed in front of the bar code reader. In contrast, the handheld type must be brought to the object in order to read the bar code. The pen type bar code reader is the most common example of a handheld bar code reader. The pen type has an optical tip which must be physically drawn across the bar code in order to read it. Another variation of the handheld type is the beam scanning bar code reader which has recently been gaining in popularity. The beam scanning reader includes a frame formed in the shape of a pistol. Inside the frame is a laser beam source for generating a laser beam, a laser beam scanning device for causing the laser beam to scan across a bar code to be read, and a detecting arrangement including beam receiving elements for receiving laser light reflected from the bar code surface. The detecting arrangement converts the reflected light into an electric signal indicative of the amount of light received. Identifying means interprets the electrical signal so as to "read" the bar code information.
A beam scanning system can detect a small spot from a distance, thus it allows the reading of both proximate and remote bar codes. In addition, the beam automatically scans the object so that no touch-and-move operation is required. In contrast, the pen type must have its cylindrical end portion touching the object to read it. The beam scanning bar code reader only require that the object be held by the hand (for the standing type), or that the bar code be sighted as a target (for the handheld type).
The beam reflected from the bar code surface includes two components: a specularly reflected beam, and a diffusely reflected beam. The amount of light derived from the specularly reflected beam is so intense that changes in the reflected light cannot be differentiated. For this reason, the beam scanning type bar code reader uses the diffusely reflected beam for reading bar codes. However, even in bar code readers which detect the diffusely reflected beam, it often happens that the specularly reflected beam also reaches the beam receiving element during the scanning operation. For example, if black characters or symbols appear on a region which is white, the intensity of the specularly reflected beam from the black characters or symbols becomes greater than that of the diffusely reflected beam from the white region. As a result, the contrast becomes poor and the bar code cannot be read. Thus, there is a "dead angle" below which a bar code cannot be read.
To overcome this problem, one alternative is to provide indirect illumination such as with touch type bar code readers which use charge coupled devices (CCDs). However, another solution must be found for beam scanning bar code readers because direct lighting is required to scan distant bar codes.
Another alternative is to restrict the angle at which the bar code reader is used relative to the bar code (such that the specularly reflected beam cannot reach the receiving element). However, this makes use of the device more inconvenient.